Axial flow compressor



June 3, 1958 G. E. CHAFMAN 2,837,270

AXIAL FLOW COMPRESSOR Filed July 24, 1952 2 Sheets-Sheet l Inventor v i7/ man By [5a 5624/.

June 3, 1958 V G. E. CHAPMAN 7,

AXIAL FLOW COMPRESSOR Filed July 24, 1952 2 Sheets-Sheet 2 u-s orCONSTANT :rr/c/mcy HUN 7 HHHW I nventor By @2567) a a? zzazz W 9 MAttorneys United States Patent 2,837,270 AXIAL FLOW COMPRESSOR GilbertE. Chapman, Indianapolis, Ind assignor to General Motors Corporation,Detroit, Mich, a corporation of Delaware Application July 24, 1952,Serial No. 3%,714 7 Claims. (Cl. 230115) My invention relates tomultistage axial flow compressors for gas turbine engines and the likeand, more particularly, to apparatus for improving both the surgecharacteristics and design speed efliciency of axial flow compressors.

Multi-stage axial flow compressors have an inherent limit of pressureratio or surge limit which varies with the speed of the compressors andwith other conditions. When the back pressure of such a compressorinstalled in a gas turbine engine reaches the surge limit, the flowthrough the compressor breaks down. The characteristics of an axial flowcompressor in a gas turbine engine are matched with the characteristicsof turbine in order to obtain maximum efiiciency at normal operating ordesign speed conditions. The compressor thus may be overloaded at lowerspeeds which must be passed through in accelerating the engine tooperating speed, or

even under starting conditions.

Even if the operating characteristic is below but very near the surgelimit, it may be difiicult to accelerate the engine Without inducingsurge and consequent breakdown of flow. Since the breakdown incompressor performance appears to be primarily a result of too low anaxial velocity component of air flow in the earlier stages and too highan axial velocity component of flow in the later stages, under these lowspeed or starting conditions surge may be eliminated by bleeding airfrom intermediate stages of the compressor to increase the air flow andhence the axial velocity through the initial stages relative to that inthe later stages.

The above may be understood more readily by referring to Fig. 4 hereinwhich depicts the characteristic curve of such a compressor andrepresents the operating line B and the surge limit line A at whichsurging of the compressor is encountered. The operating line B and surgelimit line A are plotted in terms of the ratio of discharge to inletpressure ratio (Rc) against the corrected air flow WW through thecompressor, where the quantity (theta) is compressor inlet temperature(degreesRankine) divided by 518.4 and 5 (delta) is compressor inletpressure in inches of mercury divided by 29.92. The family of generallyvertical lines represents corrected speed, N/ /(7.

For the characteristic curve illustrated, the operating line B in thelow and high speed ranges lies below the surge limit line A and stableoperation ensues. intermediate speed range, however, the surge limitline lies below the operating line, and highly erratic and unstableoperation resulting in surging of the compressor is experienced. As aresult, the engine cannot accelerate itself from the low speed range tothe normal operating range. By bleeding a sufiicient quantity of airfrom the compressor over this critical speed range, it is possible toraise the surge limit line and lower the operating line so as to obtainstable operation over the entire speed range of the compressor. I I

Another phenomenon that is particularly undesirable at or near the fulldesign speed of an axial flow compressor is the accumulation along thewalls of the'compressor casing ofa, stagnant or slow moving boundarylayer of'air, the thickness of which increases progressive- In the 2 1ytoward the discharge end of the compressor. This layer of air tends todecrease the efiective area of the discharge end of the compressor andresults in a noticeable impairment of the efiiciency thereof. The efiectof the boundary layer can be substantially mitigated, however, byaccelerating the flow of the boundary layer.

The present invention is directed to a combined compressor air bleed andboundary layer control apparatus and has for its general objectiveimproving both the surge characteristics and the design speed efliciencyof an axial flow compressor.

In accordance with my invention, both of the above functions areperformed by the same apparatus comprising an arrangement of valvedplenum chambers or manifolds mounted on the compressor casing andcommunicating with the interior of the compressor at selected low andhigh pressure stages thereof. A control system, which includes amulti-position speed sensitive switch, is provided to control thesettings of the manifold valves in such a manner as to permit asufficient quantity of air to be'bled to the atmosphere from selectedlow and high pressure stages of the compressor over the critical speedrange thereof, thereby to control the surge characteristics of thecompressor. When the compressor has attained its normal design speed,the manifold valves are arranged to permit bleeding or removalof theslowmoving boundary layer air from a high pressure section of thecompressor and reinserting or blowing this air at high velocity into theboundary layer at a low pressure section of the compressor, therebyaccelerating the boundary layer and mitigating the eifects thereof. Thecontrol system also includes apparatus responsive to temperature, say,at the inlet, of the compressor for modifying the operation of the speedsensitive switch so as to compensate for the effects of changes in thecompressor temperature on the compressor characteristics.

Other objects, features and advantages of the invention will more fullyappear from the following detailed description and drawings wherein:

Fig. 1 is a fragmentary, longitudinal sectional view of an axial flowcompressor employing-an air bleed and boundary layer control system inaccordance with the present invention; 1

Fig. 2 is a fragmentary, transverse sectional View taken in the plane 22of Fig. 1;

Fig. 3 is anenlarged view of a fragmentary portion of Fig. 1illustrating the arrangement for removal of air from the interior of thecompressor in accordance with a preferred embodiment of the invention;

Fig, 4 is a characteristic curve depiciting the operation of a highpressure multi-stage axial flow compressor; and

Fig. 5 is a schematic representation of a form of control systemsuitable for use with the present invention.

Referring to the drawings, Fig. 1 is a fragmentary showing of amulti-stage axial flow compressor embodying the present invention andcomprising a stator 10 and rotor 12. Only that portion of the compressornecessary to understanding the principles of the invention is shown. Thestator 10 may be composed of two axially disposed casing members 16 and18 which confine the low and high pressure sections, respectively, ofthe compressor and mount a number of axially spaced rings of statorvanes 22 on the interior thereof. The rotor 12, which is enclosed by thestator casing membars 16 and 18, comprises a number of spaced disks 24mountedbetween a pair of end driving wheels (not shown), the wheels anddisks being suitably secured together so as to rotate as a unit. Each ofthe wheels and disks mounts a ring of rotor blades 26 about theperiphery thereof which cooperate with the adjacent rows of stator vanesto form successive axial stages of the compressor.

Each of the compressor casing members 16 and 18 is constituted by a pairof semi-cylindrical sections provided with longitudinal bolting flanges28 as shown in Fig. 2. The adjacent ,ends ,of the halfsections ofthecasing members 16 and 18 are provided with bolting .flanges .30 aboutthe periphery thereof as shown in Fig. .1'. Axially spaced along theinner walls of the casing members 16 and 18 are a number ofcircumferential grooves 32 for mounting the stator rings. The statorvanes 22' of each of the stator rings are mounted between an outershroud band 34 and an inner shroud band 36. Each of the outer shroudbands 34 is formed with stub flanges 37, 38 (Figs. 1 and 3) which areseated between the side walls 39, 40 (Fig. 3) of the circumferentialgrooves 32 in the casing members 16 and 18.

Each half of the compressor constituted by the semicylindrical portions.of the casing members 16 and 1-8 has Welded or otherwise secured to theexterior thereof a fabricated sheet metal structure comprising a pair ofaxially spaced manifolds or plenum chambers 42', 44

which are joined by a tubular conduit 46 having a pair of exhaust ports48, 50 near the opposite ends thereof. A by-pass butterfly valve 52locatedcentrally of the conduit member 46 and a pair of similar valves54, 56 in the exhaust ports 48' and 50, respectively, are pro vided topermit air to be blown from one manifold to the other or to exhaust airexternally of the compressor from both manifolds in accordance with thesetting of the valves.

Air is extracted from the interior of the compressor into the manifolds42, 44 around the circumference of selected low andhi'gh pressure stagesof the compressor through a series of slots 60 (Figs. 2 and 3) which areformed by chamfering portions of ,the' trailing edge or side wall 40along thecircumferential length of the selected grooves and by removingcorresponding sections of the after leg or flange 38 of the outer shroudband 34. Air is thus caused to flow into the annular space behind theouter shroud band 34 and the inner wall of the selected ring grooveswithout interfering with the principal flow of air through thecompressor. In order to provide suflicient flow area, the depth of theselected ring grooves may be increased as shown in Fig. 3'. A series ofspaced ports or openings 62 formed in the compressor casing connects theannular space behind the stator rings of the selected grooves with therespective manifolds 42 and 44. Thelocation of the manifoldsAZ and 44 isnot critical. For example, however, in a sixteen stage compressor, Iprefer tolocate them at the outlet of the fourth and twelfth stages.

The valvesSZ; 54' and 56 may be operated by solenoids or other actuatorsenergized by a control system, a simple form of which is illustrateddiagrammatically in Fig. 5. The control system includes speed sensitivemeans such as a speed responsive switch-70 which comprises a rotatableshaft portion 72 coupled to the compressor rotor shaft 15 throughsuitable gearing 74- and a conducting rod or slider portion'76 coupledto the shaft portion 72 through a fiyball governor 78 that varies theaxial position of the conducting rod in accordance with changesincompressor speed. A suitable source of electric power, such as a battery80, is connected through a switch 82 to the-conducting rod 76 which hasa pair of spaced sliding contacts 84, 86 hearing on the surface thereof.The contact 84 is connected through line 88 to the armature coils 90, 92of the normally closed solenoid-opened valves 54, 56, respectively,while contact 36 is connected. through line 93 to armature coil 94 ofnormally open solenoid valve 52'. Two insulating segments 96, 98provided on the conducting rod 76 serve to break the. electricalcircuits of the valves as the compressor is accelerated or deceleratedthrough its operating range. Slider'76 is illustrated in the zero speedposition and'moves downward in Fig. as the, compressor: speed increases;

The operation of the control system is as follows:

Switch 32 is closed to place the control system in operating conditionand the compressor is started with all valves closed, the coils and 92of the solenoid valves being de-energized under starting conditions, andthe coil 94 being energized. The characteristics of the compressorfollow the operating line B of Fig. 4 as the speed of the machineincreases. At a predetermined speed corresponding to point 1 on theoperating line, the circuit of the exhaust valves 54 and 56 is closed asthe insulating segment 98 moves' from under contact 84 and the valves54, 56 are opened to bleed air from the compressor. Stable operationthen occurs as the speed of the compressor'increases; Slightly beyondthe critical speed range at a predetermined 'speed cor responding topoint g on line B, the circuit of-valves 54 and 56 is again opened bythe insulating segment 96 and the valves are caused to close. Thus,stable operation has been provided through the critical speed range bymeans of the air bleed arrangement.

At all speeds above that corresponding to point It, the by-pass valve 52is open as insulating segment .98' moves under contact 86 to de-energizesolenoid 94 so that air flows from the high pressure end of the compressor to the low pressure end. The boundary layer at the high pressureend is accelerated by bleeding or removing the slow-moving air thereof,and is accelerated at the low pressure end by the blowing action of thehigh pressure air as this air returns to the low pressure end of thecompressor. Thus, the boundary layer is prevented from buildingup alongthe compressor casing and the compressor efficiency is improved.

The valves operate in a reverse order during the dc celeration of thecompressor. Depending upon the compressor characteristics, it may bedesirable to maintain the valves 54 and 56 open during startingconditions from zero speed until the point g is reached.

In order to compensate for the effects of changesin the compressor inlettemperature, for example, on'-the compressor surge and operatingcharacteristics, the op- 1 eration of the speed sensitive switch 70 maybe modified in accordance with my invention to maintain the selectedpositions of f, g and h, irrespective of the value of cont I pressorinlet temperature and hence #5; This is -a'ccomplished by provision of atemperature probe 100 in the inlet of the compressor as shown in Fig; 5.The temperature probe is connected to a pressure respon sive device suchas a Sylphon bellows1'92 which ad jllSiS the position 'of a pair ofcamming surfaces1'04;-

106 to change the position of the slide contacts 84 'an'd 86 relative tothe initial position of rod 76.

The cams 1M and 106 are calculated to compensate Y for the \/5 factor inthe N/ term of the characteristic curve of Fig. 4. While thecompensation is'not rigoronsly' exact, it issufficient for allpracticalpurposes.

Although a specific embodiment of my inventionhas been shown anddescribed, it will be understood thatit is but illustrative and thatvarious modifications may be made therein without departing from thespirit and scope of the invention.

I claim:

1. In combination, a multi-stage axial flow compressor comprising astator and'a rotor, said stator including groove and its respectiveshroud band, ports being formed in said compressor casing communicatingwith said selected grooves, said compressor casing having manifold meansthereon connected to said ports.

2. In combination, a multi-stage axial flow compressor comprising astator and a rotor, said stator including an outer casing surroundingsaid rotor and containing a plurality of axially spaced circumferentialgrooves within the interior thereof, each of said grooves having aninner wall and a pair of spaced side walls, a ring of stator vanes foreach of said grooves including an outer shroud band and a plurality ofradially extending stator vanes mounted therein, said shroud band beingseated between the side walls of respective ones of said grooves,portions along the circumferential length of one of the side walls ofselected ones of said grooves being chamfered to form a plurality ofspaced slots between each of said selected grooves and its respectiveshroud band, ports being formed in said compressor casing communicatingwith said selected grooves, and manifold means mounted on saidcompressor casing connected to said ports.

3. In combination, a multi-stage axial flow compressor comprising astator and a rotor, said stator including an outer casing surroundingsaid rotor and containing a plurality of axially spaced circumferentialgrooves within the interior thereof, each of said grooves having aninner wall and a pair-of spaced side walls, a ring of stator vanes foreach of said grooves including an outer shroud band and a plurality ofradially extending stator vanes mounted therein, said shroud band beingseated between the side walls of respective ones of said grooves,portions along the circumferential length of one of the side walls ofselected ones of said grooves being chamfered and corresponding portionsof the shroud bands seated in said selected grooves being removed toform a plurality of spaced slots between each of said selected groovesand its respective shroud band, ports being formed in said compressorcasing communicating with said selected grooves, and manifold meansmounted on said compressor casing connected to said ports.

4. In combination, a multi-stage axial flow compressor comprising astator and a rotor, said stator including an outer casing surroundingsaid rotor and containing a plurality of axially spaced circumferentialgrooves within the interior thereof, each of said grooves having aninner wall and a pair of spaced side walls, a ring of stator vanes foreach of said grooves including an outer shroud band and a plurality ofradially extending stator vanes mounted therein, said shroud band beingseated between the side walls of respective ones of said grooves,portions along the circumferential length of one of the side walls of atleast one of said grooves being chamfered to form a plurality of spacedslots between said groove and its respective shroud band, ports beingformed in said compressor casing communicating with said groove, exhaustmeans mounted on said compressor casing connected to said ports, conduitmeans connected between said ports for conducting air from high pressureto low pressure stages of said compressor, valve means in said exhaustmeans and in said conduit means, and speed responsive control means forcontrolling the position of said valve means over the operating speedrange of the compressor.

5. In combination, a multi-stage axial flow compressor comprising astator and a rotor, said stator including an outer casing surroundingsaid rotor and containing a plurality of axially spaced circumferentialgrooves within the interior thereof, each of said grooves having aninner wall and a pair of spaced side walls, a ring of stator vanes foreach of said grooves including an outer shroud band and a plurality ofradially extending stator vanes mounted therein, said shroud band beingseated between the side walls of respective ones of said grooves,portions along the circumferential length of one of the side walls of atleast one of said grooves being chamfered to form a plurality of spacedslots between said one groove and its respective shroud band, portsbeing formed in said compressor casing communicating with said groove,exhaust means mounted on said compressor casing connected to said ports,conduit means connected between said ports for conducting air from highpressure to low pressure stages of said compressor, valve means in saidexhaust means and in said conduit means, and control means includingspeed responsive means for controlling the position of said valve meansover the operating range of the compressor, means responsive totemperature in the compressor, and means operatively connected with saidtemperature responsive means and said speed responsive means andaffecting operation of said valve means in accordance with the effect oftemperature changes in the operating characteristics of the compressor.

6. A multi-stage axial flow compressor compressing, in combination, arotor and a stator having a casing surrounding the rotor, said statorcasing having a plurality of axially spaced circumferential groovesformed within the interior boundary thereof and further having aplurality of openings extending therethrough and communicating with atleast one of said grooves therein, a ring of stator vanes for each ofsaid grooves, each of said rings including an outer shroud band seatedin a respective one of said grooves and a plurality of radiallyextending stator vanes mounted in each of said bands, said one of saidgrooves having portions of the edge thereof relieved to form a pluralityof spaced slots between said one groove and its respective shroud band,and exhaust means on said compressor casing communicating with theinterior of the compressor through said ports and slotted grooves.

7. In combination, a multi-stage axial flow air compressor comprising arotor and a stator having a casing surrounding the rotor, said statorcasing having at least two axially spaced ports formed thereincommunicating interiorly with the compressor at a low pressure stage andat a high pressure stage respectively, exhaust conduit means connectingeach of said ports to atmosphere for exhausting air from said stages,by-pass conduit means connecting across said exhaust conduit means forconducting air from said high pressure stage to said low pressure stage,separate valve means in both of said exhaust conduit means and in saidby-pass conduit means, the compressor being subject to surgeatintermediate speeds unless air bled to atmosphere at such speeds andbeing subject to the accumulation of a substantially stagnant boundarylayer of air along the inner wall of said stator casing at high speedsunless the boundary layerof air is accelerated in some fashion at suchspeeds, and an automatic control including compressor speed sensitivemeans for controlling the position of said three separate valve means,said control including first means maintaining both of said exhaustvalve means open and said bypass valve means closed at intermediatespeeds whereby said surge is avoided by air bleeding said stages throughsaid exhaust conduit means and said control including second meansmaintaining both of said exhaust valve means closed and said 'by-passvalve means open at high speeds whereby said accumulation of a stagnantboundary layer of air is avoided by transferring air from the boundarylayer at said high pressure stage to the boundary layer at said lowpressure stage through said by-pass conduit means.

References Cited in the file of this patent UNITED STATES PATENTS

